This invention relates to the production of glass fibers, e.g., fibers made by melting particulate batch ingredients or minerals, including basalt and the like, and, more particularly, to a method of and apparatus for controlling the thickness of a blanket of batch floating on a body of molten glas in a glass melting furnace and for controlling the level of molten glass in such furnace.
Electric glass melting furnaces have a plurality of submerged electrodes that are positioned in the furnace in a predetermined pattern. An electric current is caused to flow through the molten glass between the electrodes to heat the glass by the Joule effect. Glass batch material is supplied to the top surface of the molten glass to provide both a source of supply and an insulating layer or blanket. The amount of batch supplied to the furnace must be carefully controlled to insure that a blanket of predetermined thickness is maintained over the entire top of the furnace. This blanket of batch reduces heat loss from the furnace and also protects the feeder from excessive heat, since the amount of heat lost is inversely proportional to the thickness of the batch. A constant batch thickness increases the life of the furnace by reducing the stress and warpage of the furnace that are caused by temperature fluctuations.
Maintaining a constant level of molten glass in the furnace is also important, since a constant glass level reduces the erosion of the refractory and thus extends the life of the furnace. In addition, it provides a constant head pressure and a uniform glass temperature, resulting from a uniform dwell time in the furnace; both of which assist in the production of fibers of uniform diameter. Still further, a constant glass level ensures that the glass level will not drop below the level of the heating element in furnaces that do not have bottom entering heating elements.
The prior art has attempted to control the thickness of the blanket of batch by remotely sensing the temperature of the surface of the batch, such as by using an infrared sensor. However, it has been found that outgassing, which causes eruptions in the surface of the batch cover at or near the area that the infrared sensor is monitoring, causes the thickness of the batch cover to drift from the desired set point. This drifting problem is intensified when only a thin blanket of batch, such as 3/8 to 1 inch (0.95 to 2.54 centimeters), is maintained on the surface of the molten glass. Moreover, the prior art has not provided a method of controlling the level of molten glass in a furnace at a predetermined level. Rather, the prior art has allowed the level to vary, thereby affecting the quality of the fibers produced and shortening the useful life of the furnace.
Therefore, it is an object of this invention to provide a reliable and accurate method of and apparatus for controlling the thickness of a blanket of batch material floating on a body of molten glass in a glass melting furnace. It is an additional object of the present invention to provide a method of precisely maintaining the level of the molten glass at a predetermined level.